This invention relates to an optical element which is used to effect imaging by irradiation of a vacuum using ultraviolet radiation or X-radiation and a projection exposure apparatus employing the optical element, and more particularly to a reflection mask for X-ray reduction lithography for use for pattern replication of a semiconductor device and a projection exposure apparatus employing the reflection mask.
Refinement of a circuit pattern has been and is proceeding in order to enhance the degree of integration and the response speed of an LSI solid state device. A reduction projection exposure method which employs ultraviolet radiation for exposure is used popularly for the formation of a circuit pattern at present. The resolution in the method increases in proportion to the exposure wavelength .lambda. but in inverse proportion to the numerical aperture NA of the projection optical system. Enhancement of the resolution limit has been achieved by increasing the numerical aperture NA. This method, however, is approaching its limit due to a decrease of the depth of focus and the difficulty in designing and manufacturing the reflection optical system (lens or lenses).
Therefore, there is a current attempt to shorten the exposure wavelength .lambda.. The shortening of the exposure wavelength .lambda. can be seen, for example, from the change-over from the g line (.lambda.=435.8 nm) to the i line (.lambda.=365 nm) of a mercury lamp and further to a KrF excimer laser (.lambda.=248 nm). The resolution is enhanced by shortening of the exposure wavelength. However, from a limit in principle originating from the magnitude of a wavelength of ultraviolet radiation used for exposure, it is difficult to obtain the resolution of 0.1 .mu.m or less using a conventional exposure technique.
Meanwhile, one of the methods of forming a refined pattern is proximity X-ray lithography which employs soft X-rays having an exposure wavelength of 0.5 nm to 2 nm. This method may probably provide a high resolution of 0.1 .mu.m or less in principle since the exposure wavelength is short.
Generally, in order to form a circuit pattern, a mask pattern is replicated on a resist on a wafer. In proximity X-ray lithography, a transmission mask called a proximity X-ray mask is employed. A portion of the proximity X-ray mask through which an X-ray is to pass is constituted from a thin film, or membrane, formed from a light-element material such as Si, SiN or C and normally having a thickness of 2 .mu.m or so. At another portion of the proximity X-ray mask at which an X-ray is absorbed, a circuit pattern called an X-ray absorber of a heavy metal such as W, Au or Ta having a thickness of 0.5 .mu.m to 1.0 .mu.m or so is formed on the membrane.
Accordingly, a proximity X-ray mask has the problem that, since a circuit pattern is formed on a membrane having very low rigidity, the circuit pattern is sometimes deformed by an internal stress of the heavy metal of the X-ray absorber, an external force acting thereupon when the X-ray mask is loaded onto a predetermined exposure apparatus or by some other cause, and consequently, the desired circuit pattern cannot be replicated on a resist on a wafer. Particularly in proximity X-ray lithography, since a pattern of a proximity X-ray mask is replicated equally at the multiplying factor of 1, deformation of the pattern on the proximity X-ray mask is replicated also equally at the multiplying factor of 1. The problem of deformation of a pattern on a proximity X-ray mask having low rigidity is a serious subject in proximity X-ray lithography.
On the background described above, attention has been paid in recent years to X-ray reduction lithography, which employs vacuum ultraviolet radiation or soft X-rays as an exposure source. The X-ray reduction lithography is disclosed, for example, in Japanese Journal of Applied Physics, Vol. 11B, No. 30, 1991, p. 3,051.
FIG. 1 shows an example of an exposure optical system for X-ray reduction lithography. Vacuum ultraviolet radiation or soft X-radiation 411 is employed as an exposure source and is introduced obliquely at an incidence angle .theta. to irradiate a reflection mask 81. While the incidence angle .theta. may differ among different optical systems, it ranges from 1.degree. to 15.degree. or so. In order to prevent radiation reflected from a mask from being obscured by a reflection optical system, an exposure optical system for X-ray reduction lithography does not allow the incidence angle of 0.degree. (normal incidence).
The reflection mask 81 has a multilayer 21 formed thereon which can regularly reflect the vacuum ultraviolet radiation or soft X-rays 411. A predetermined pattern is formed on the multilayer 21. The vacuum ultraviolet radiation or soft X-rays 411 reflected from the reflection mask 81 are then reflected by a convex mirror 92 and then by a concave mirror 91 and imaged on a wafer 82. A multilayer 7 is formed on each of the convex mirror 92 and the concave mirror 91. Generally, when an xyz coordinate system is employed with such an optical system as described above and as shown in FIG. 1, the x direction is called the tangential direction, and the y direction is called the sagittal direction. In order to expand an exposure field of a mask and an exposure area of a wafer in such an optical system as shown in FIG. 1, the mask is sometimes scanned in the tangential direction in synchronism with the wafer.
In accordance with the invention, a multilayer which is used as a reflecting mirror for vacuum ultraviolet radiation or X-rays is a film wherein at least two different substances having different refraction factors are layered alternately with the thickness thereof adjusted so as to make a predetermined cycle, and the reflectivity of the multilayer against the vacuum ultraviolet radiation or the X-rays depends upon the number of layer pairs of the film when the wavelength and the incidence angle of the vacuum ultraviolet radiation or the X-rays are fixed.
For example, the relationship between the number of layer pairs of Ni and C and the reflectivity of a Ni/C multilayer when nickel (Ni) having a thickness of 1.27 nm and carbon (C) having a thickness of 1.27 nm are alternately layered each by 200 layers (by 200 cycles with the cycle length of 2.54 nm) to form the Ni/C multilayer is shown in FIG. 2. Here, the wavelength of an X-ray used to illuminate the multilayer is 5 nm, and the incidence angle of the X-rays is 10.degree..
The reflectivity of the multilayer increases as the number of layer pairs of Ni and C which contribute to reflection increases, and finally becomes saturated with about 160 layer pairs (cycles) or so. Accordingly, when the wavelength of an X-ray used for exposure is 5 nm and the incidence angle of the X-ray is 10.degree. using a Ni/C multilayer as a reflecting mirror, a thickness of about 0.4 .mu.m or more is necessary in order to obtain effective reflectivity.
A conventional reflection X-ray mask is normally constructed such that, as disclosed, for example, in Extended Abstracts of the 18th Conference on Solid State Devices and Material, 1986, pp. 17-20, a region in which it has a comparatively high reflectivity with regard to vacuum ultraviolet radiation or X-rays is formed as a portion in which a multilayer exists, while another region in which it has a comparatively low reflectivity or has a reflectivity substantially equal to zero is formed as a portion in which no multilayer or no multilayer structure exists.
In the reflection mask shown in FIG. 3, a multilayer 2 on a substrate 1 is removed at a certain portion thereof to form a low- or no-reflectivity region 3 in which the reflectivity is low or substantially equal to zero. The portion at which the multilayer 2 remains is a high-reflectivity region. A pattern is formed on the reflection mask by an arrangement of the low- or no-reflectivity regions 3 and the high-reflectivity regions.
FIG. 4 shows the relationship between the position of the pattern of a reflection mask and the reflected light intensity of incident radiation when, using the reflection mask shown in FIG. 3, vacuum ultraviolet radiation or X-radiation 411 is introduced as incident exposure at a predetermined incidence angle (.theta.&gt;0.degree.) into the reflection mask. Here, side faces of a left-hand side boundary 45 and a right-hand side boundary 47 of the pattern of the reflection mask extend perpendicularly to the surfaces of the substrate.
The incident radiation 411 is introduced obliquely at the incidence angle .theta. (&gt;0.degree.) from the left into the mask. Reflected radiation 41 has an ordinary intensity. Where the thickness of the pattern is represented by a, the total number of the layered films of the multilayer which contribute to reflection of the incident radiation 411 is decreased at the location spaced by 2a.cndot.tan.theta. from the left-hand side boundary 45 of the pattern 22. Consequently, the intensity of the reflected radiation 43, that is, the reflectivity, decreases at the location spaced by 2a.cndot.tan.theta. from the left-hand side boundary 45 of the pattern.
Further, there is another problem that, when the radiation 411 is incident from the left in FIG. 4, the reflected radiation escapes at the location spaced by a.cndot.tan.theta. from the right-hand side face 47 of the pattern-like reflected radiation 44. Accordingly, since the intensity of reflected radiation decreases at the left-hand side boundary 45 of the pattern while the intensity of reflected radiation exists on the outer side of the width T of the pattern at the right-hand side boundary 47 of the pattern, a problem takes place in imaging and replication of a mask pattern of a desired size.
Another reflection mask is disclosed in Japanese Patent Laid-Open Application No. 1-152725 and is shown in FIGS. 5(a) and 5(b). Referring to FIG. 5(a), the surface of a substrate 1 of a reflection mask is removed in advance by etching to make concave structures 33 to form a predetermined pattern in which the concave structures 33 serve as non-reflecting portions. A multilayer reflecting mirror is then formed on the surface of the substrate so as to form reflecting portions and non-reflecting portions 34 from the convex (unetched) structure portions and the concave structure portions, respectively (FIG. 5(b)).
The manufacturing process of the reflection mask is advantageous in that, since a multilayer is formed after a pattern has been formed in advance, the opportunity for a defect to take place in the multilayer in the manufacturing process is small. However, since no attention is paid to the angles of a left-hand side boundary 45 and a right-hand side boundary 47 of a pattern with respect to incident radiation, the reflected radiation intensity is lower at the boundaries of the pattern similarly as in the mask shown in FIG. 4.
FIG. 6 shows the relationship between the position of the pattern and the reflected radiation intensity. Since the intensity of reflected radiation decreases at the left-hand side boundary 45 of the pattern while the intensity of reflected radiation exists on the outer side of the width T of the pattern at the right-hand side boundary 47 of the pattern, a problem takes place in imaging and replication of a mask pattern of a desired size.
A further reflection mask is disclosed in Japanese Patent Laid-Open Application No. 64-4021 wherein, as shown in FIG. 7, absorber patterns 35 each having a predetermined thickness and a predetermined profile are formed as non-reflecting portions on a multilayer 2. With the mask structure, however, since the absorber patterns 35 have a predetermined thickness b, reflecting portions and non-reflecting portions have a step therebetween. FIG. 8 shows the relationship between the position of the pattern and the reflected radiation intensity. Similarly as for the two masks described above, on the left-hand side of each pattern, the total number of those layers of the multilayer which contribute to reflection of the incident radiation 411 is decreased at the location spaced by 2a.cndot.tan.theta. from the left-hand side boundary 45 of the pattern, and the intensity of the reflected radiation 43, that is, the reflectivity, decreases there. Further, since a shadowed portion 36 is produced in a range of b.cndot.tan.theta. due to the thickness b of the absorber pattern 35, the reflectivity of the pattern having a predetermined width is decreased. Since, in the relationship between the position of the pattern and the reflected radiation intensity shown in FIG. 8, the intensity of reflected radiation decreases at the boundary of the pattern while the intensity of reflected radiation exists on the outer side of the width T of the pattern, a problem takes place in imaging and replication of a pattern of a reflection pattern.
The necessary thickness of the absorber pattern 35 of the reflection mask shown in FIG. 8 is 0.2 .mu.m when Cr is used for the absorber and is 0.1 .mu.m when Au is used for the absorber, where the wavelength of an X-ray as incident radiation is 5 nm and the contrast is equal to or higher than 50. The thickness of any material used for the absorber must be increased as the wavelength of the vacuum ultraviolet radiation or X-radiation used for irradiation of the mask decreases.
FIGS. 9(a) and 9(b) show the positional relationship between an exposure optical system and a reflection mask 81 when a pattern 2222 of the reflection mask 31 is imaged and replicated by means of the exposure optical system described hereinabove with reference to FIG. 1. FIG. 9(a) is a sectional view of the mask, and FIG. 9(b) is a plan view. The intensity of reflected light decreases on a side face 361 of the pattern which extends in parallel to the sagittal direction.